Molecular Spectroscopy Laboratory

Outline

Spectroscopy is the “eyes” of modern science, and hence it plays essential roles in a variety of research fields including physics, chemistry and biology. We develop and utilize the most advanced spectroscopy for molecular science of complex systems in the condensed phase. To elucidate a variety of complex phenomena occurring in the condensed phase, we need to clarify the electronic and vibrational states of molecules, the response of surroundings, and the fluctuation and dissipation of energy behind. Based on this view, we carry out fundamental research using the most advanced linear/nonlinear spectroscopic methods with most suitable time- and space-resolution for the problems to be studied. Currently, we are carrying out the following projects: (1) Elucidation and control of ultrafast phenomena using advanced time-resolved spectroscopy; (2) Study of soft interfaces using new nonlinear spectroscopy; (3) Study of the dynamics of complex systems in the femtosecond – millisecond time region.

Targets of these projects 1, 2 and 3 are (1) fundamental molecules in solution, (2) molecules at air/liquid, liquid/liquid, liquid/solid and biological interfaces, and (3) biological macromolecules, respectively.

Recent Research Topic

Seeing molecules at interface by detecting phase of light

Fig. 1 Principle and apparatus of interface-selective second-order nonlinear spectroscopy: heterodyne-detected sum-frequency generation spectroscopy

It is very important to understand the behavior of molecules at interfaces not only from the viewpoint of fundamental science but also from the viewpoint of application. Because interfacial molecules are sandwiched by two totally different phases, they show significantly different properties from those in homogeneous gas, liquid and solid phases. However, the behavior of interfacial molecules has not been well clarified. Second-order nonlinear spectroscopy that utilizes intense lasers is a very powerful tool to study interfacial molecules that exist in only one or two monolayer thickness at the interface. We have developed new interface-selective nonlinear spectroscopies, utilizing advanced femtosecond laser technology. Especially, multiplex heterodyne-detected sum-frequency generation spectroscopy provides new information on electronic and vibrational states of interfacial molecules, which could not be obtained before. In this method, we irradiate interfaces with two laser pulses: one is the narrow band ω1 pulse and the other is the broad band ω2 pulse. Then, a sum-frequency signal having ω1+ω2 frequency is generated in a wide energy region at the interface and is mixed with a reference laser pulse (local oscillator) with a controlled phase to detect the interference between the signal and local oscillator (Fig. 1). With this new method, we achieved simultaneous detection of electronic or vibrational spectra of interfacial molecules without losing phase information. We measured electronic spectra of a dye molecule (coumarin) at the air/water interface and found that the spectrum at the interface appears in the energy region between the spectra observed in water and a nonpolar solvent (Fig. 2). This is a clear manifestation of the “half solvation” at the interface, which means that half of the molecule is solvated by water and the other half is exposed to the air. We also examined the structure and orientation of water at the interface by measuring vibrational spectra. We clearly observed the phase change of the nonlinear signal when the charge at the interface was changed from positive to negative, and obtained vibrational spectra having opposite signs (Fig. 3). This is a direct proof of the flip-flop model of interfacial water: the flip-flop of the orientation of the water molecule is induced by inversion of the charge at the interface. Our newly developed nonlinear spectroscopy enables us to elucidate novel phenomena at the gas/liquid, liquid/liquid, liquid/solid and biological interfaces.

Fig. 2 Electronic spectrum of a coumarin dye at the air/water interface (red)

Absorption spectra in water (blue) and a non-polar solvent (butyl ether: green) are shown for comparison. A picture of the “half solvation” obtained from molecular dynamics simulation is also shown in the inset.

Fig. 3 The vibrational spectra of charged water interfaces (the OH stretch region)

SDS and CTAB have charged head groups having opposite signs, which induces the flip-flop of the interfacial water.